Researchers from the United States and Japan have discovered details of generation-long droughts in Indochina going as far back as 700 years by studying tree ring core samples from Fokienia hodginsii, a rare species of ancient conifers that grows in Vietnam’s cloud forests, Reuters reported.

They discovered two persistent drought periods in the early 1400s, which coincided with the collapse of the large and wealthy Angkor civilization in what is now Cambodia.

“There was a very significant multi-decadal drought in the early 1400s with the worst drought year being 1417,” said Brendan Buckley of the Tree Ring Laboratory at the Lamont-Doherty Earth Observatory in the United States.

A third major drought lasting at least 30 years occurred in the mid-18th century, said Buckley, who worked with Masaki Sano and Tatsuo Sweda of Ehime University of Japan studying the tree rings of Fokienia.

“All of the kingdoms in Southeast Asia collapsed, in Thailand, Vietnam and Laos between 1750-80,” he said.

“The research is helping unravel the complexity of the annual monsoon that usually begins during March-May and on which millions rely to grow crops, particularly rice in the Mekong Delta. It could also help understand how climate change could affect the densely populated region and its economies.” Reuters reported.

“Buckley said the chronology constructed from 36 conifer trees found in a national park near Dalat showed a strong correlation between dry spells and the El Nino weather pattern that typically brings drought to Southeast Asia and eastern Australia.”

“Fokienia is an exceptional tree species because of the way it grows and responds to drought,” he said, adding the data was corroborated with a lot of historical records, such as ancient lanna palm leaf texts from Thailand and accounts of foreign merchants, plus tree-ring data from teak trees in Thailand.

“What the Fokienia trees are most keenly tuned into is the length of the monsoon. Longer the monsoon, the trees grow more.”